Water management in fuel cells

ABSTRACT

An electrochemical fuel cell having an anode, an ion transfer membrane and a cathode has liquid water delivered to the fluid flow channels within the cathode so as to maintain a relative humidity of 100% throughout the fluid flow channels. A calibration method and apparatus is described for determining an optimum quantity or range of quantities of liquid water to be delivered to the cathode fluid flow channels under varying operating conditions. An operating method and apparatus is described that ensures an optimum quantity of liquid water is delivered to the cathode fluid flow channels under varying operating conditions.

The present invention relates to electrochemical fuel cells, such assolid polymer electrolyte fuel cells, which convert fuel and oxidantinto electrical energy and a reaction product.

A typical layout of a conventional fuel cell 10 is shown in FIG. 1which, for clarity, illustrates the various layers in exploded form. Asolid polymer ion transfer membrane 11 is sandwiched between an anode 12and a cathode 13. Typically, the anode 12 and the cathode 13 are bothformed from an electrically conductive, porous material such as porouscarbon, to which small particles of platinum and/or other precious metalcatalyst are bonded. The anode 12 and cathode 13 are often bondeddirectly to the respective adjacent surfaces of the membrane 11. Thiscombination is commonly referred to as the membrane-electrode assembly,or MEA.

Sandwiching the polymer membrane and porous electrode layers is an anodefluid flow field plate 14 and a cathode fluid flow field plate 15.Intermediate backing layers 12 a and 13 a may also be employed betweenthe anode fluid flow field plate 14 and the anode 12 and similarlybetween the cathode fluid flow field plate 15 and the cathode 13. Thebacking layers are of a porous nature and fabricated so as to ensureeffective diffusion of gas to and from the anode and cathode surfaces aswell as assisting in the management of water vapour and liquid water.

The fluid flow field plates 14, 15 are formed from an electricallyconductive, non-porous material by which electrical contact can be madeto the respective anode electrode 12 or cathode electrode 13. At thesame time, the fluid flow field plates must facilitate the deliveryand/or exhaust of fluid fuel, oxidant and/or reaction product to or fromthe porous electrodes. This is conventionally effected by forming fluidflow passages in a surface of the fluid flow field plates, such asgrooves or channels 16 in the surface presented to the porous electrodes12, 13.

With reference also to FIG. 2( a), one conventional configuration offluid flow channel provides a serpentine structure 20 in a face of theanode 14 (or cathode 15) having an inlet manifold 21 and an outletmanifold 22 as shown in FIG. 2( a). According to conventional design, itwill be understood that the serpentine structure 20 comprises a channel16 in the surface of the plate 14 (or 15), while the manifolds 21 and 22each comprise an aperture through the plate so that fluid for deliveryto, or exhaust from, the channel 16 can be communicated throughout thedepth of a stack of plates in a direction orthogonal to the plate asparticularly indicated by the arrow in the cross-section on A-A shown inthe FIG. 2( b).

Other manifold apertures 23, 25 may be provided for fuel, oxidant, otherfluids or exhaust communication to other channels in the plates, notshown.

The channels 16 in the fluid flow field plates 14, 15 may be open endedat both ends, ie. the channels extending between an inlet manifold 21and an outlet manifold 22 as shown, allowing a continuous throughput offluid, typically used for a combined oxidant supply and reactantexhaust. Alternatively, the channels 16 may be closed at one end, ie.each channel has communication with only an input manifold 21 to supplyfluid, relying entirely on 100% transfer of gaseous material into andout of the porous electrodes of the MEA. The closed channel maytypically be used to deliver hydrogen fuel to the MEA 11-13 in a combtype structure.

With reference to FIG. 3, a cross-sectional view of part of a stack ofplates forming a conventional fuel cell assembly 30 is shown. In thisarrangement, adjacent anode and cathode fluid flow field plates arecombined in conventional manner to form a single bipolar plate 31 havinganode channels 32 on one face and cathode channels 33 on the oppositeface, each adjacent to a respective membrane-electrode assembly (MEA)34. The inlet manifold apertures 21 and outlet manifold apertures 22 areall overlaid to provide the inlet and outlet manifolds to the entirestack. The various elements of the stack are shown slightly separatedfor clarity, although it will be understood that they will be compressedtogether using sealing gaskets if required.

In order to obtain high and sustained power delivery capability from afuel cell, it is generally necessary to maintain a high water contentwithin the membrane-electrode assembly, and in particular within themembrane.

In the prior art, this is conventionally achieved by humidifying thefeed gases, either fuel, air or both, fed via manifolds 21, 22 or 23 andchannels 16. In other words, water in the vapour phase (hereinafter‘gaseous water’) is introduced into the channels 16. This also can alsocontribute, to some limited extent, to heat management within the fuelcell assembly.

Another method is to deliver water in the liquid phase (hereinafter‘liquid water’) directly to the membrane 11, 34, e.g. directly to theelectrode surfaces or into the channels 16 of the bipolar plates 31.This technique has the advantage of not only supplying the water tomaintain a high membrane water content but can also act to coolsignificantly the fuel cell through evaporation and extraction of latentheat of vaporisation. A detailed description of techniques forintroducing liquid phase water directly to the electrode surfaces orinto the channels 16 has been described in international patentapplication no. PCT/GB03/02973 (unpublished at the time of filing thisapplication). Relevant parts of that document are therefore reproducedherein where appropriate.

This direct heat removal process that provides for the extraction ofthermal energy via the exit gas stream has distinct advantagesassociated with the elimination of intermediate cooling plates withinthe fuel cell stack assembly.

It is an object of the present invention to provide a method andapparatus for providing improved operation of an evaporatively cooledfuel cell stack through introduction of excess water into the channels16 of the cathode electrode.

According to one aspect, the present invention provides a method ofoperating an electrochemical fuel cell having an anode, an ion transfermembrane and a cathode, comprising the steps of:

-   -   delivering fluid fuel to fluid flow channels within the anode;    -   delivering fluid oxidant to fluid flow channels within the        cathode;    -   exhausting reaction by-products and any unused oxidant from the        fluid flow channels within the cathode; and    -   delivering a sufficient quantity of liquid water to the fluid        flow channels within the cathode such that a relative humidity        of 100% is maintained substantially throughout the fluid flow        channels.

According to another aspect, the present invention provides anelectrochemical fuel cell assembly comprising:

-   -   at least one anode fluid flow field plate having fluid flow        channels therein;    -   at least one ion transfer membrane;    -   at least one cathode fluid flow field plate having fluid flow        channels therein;    -   means for delivering fluid fuel to the anode fluid flow        channels;    -   means for delivering fluid oxidant to the cathode fluid flow        channels;    -   a water injection mechanism for delivering a sufficient quantity        of liquid water to the fluid flow channels within the cathode        such that a relative humidity of 100% is maintained        substantially throughout the fluid flow channels during normal        operating conditions of the fuel cell.

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows a schematic cross-sectional view through a part of aconventional fuel cell;

FIGS. 2( a) and 2(b) respectively show a simplified plan and sectionalview of a fluid flow field plate of the fuel cell of FIG. 1;

FIG. 3 shows a cross-sectional view through a conventional fuel cellstack with bipolar plates;

FIG. 4( a) shows a plan view of a fuel cell fluid flow field plate witha serpentine fluid conduit, showing in outline the overlay position of awater distribution foil and cover foil;

FIG. 4( b) shows a plan view of a fuel cell fluid flow field plate withinterdigitated comb fluid conduit, showing in outline the overlayposition of a water distribution foil and cover foil;

FIG. 5 shows a plan view of a water distribution foil;

FIG. 6 shows a cross-sectional view of the fluid flow field plate, waterdistribution foil and cover foil of FIGS. 4 and 5;

FIG. 7 shows a perspective view of part of the assembly of FIG. 6;

FIG. 8 shows a cross-sectional view of a fluid flow field plate, waterdistribution foil and cover foil in which the relative positions of thewater distribution foil and cover foil are reversed;

FIG. 9 shows a schematic plan view of water injection points for aninterdigitated comb channel structure;

FIG. 10 is a schematic diagram illustrating the principles of watercooling of the cathode of a fuel cell;

FIG. 11 is a graph illustrating the variation in mass of gaseous waterper unit mass of air as a function of temperature for fully saturatedconditions, i.e. at 100% relative humidity;

FIG. 12 is a graph illustrating the variation in a fuel cell stackvoltage as a function of the flow rate of liquid phase water supplied tothe cathode;

FIG. 13 is a graph illustrating the theoretical minimum water flow raterequired as a function of fuel cell stack current; and

FIG. 14 is a schematic diagram of components of a fuel cell stack systemincluding a water delivery management system.

During operation of a fuel cell stack assembly 30, heat is generatedwithin the fuel cell stack as a consequence of electrochemical andelectrical losses. In an example of an evaporatively cooled fuel cell 10in a stack assembly, shown schematically in FIG. 10, this heat isremoved via an increase in temperature of the exhaust products 100, 101over the inlet temperature of the reactants 102, 103 and by thevaporisation of liquid water 104 supplied to the cathode 13 andevaporated into the cathode air stream 103. At all but the lowest powerlevels, evaporative cooling is found to be the dominant mechanism forheat removal.

Evaporation of liquid water 104 will occur if the partial pressure ofgaseous water in the cathode air stream is low enough, i.e. atconditions of relative humidity<100%, and there is a supply of heat tovaporise the liquid water. Once the local conditions are such that therelative humidity of water is 100%, i.e. the air is saturated with watervapour, no further evaporation will occur unless any of the followingthree conditions prevail: (i) the air flow rate is increased such thatthe partial pressure of gaseous water is lowered in inverse proportion;(ii) the total pressure is lowered such that the partial pressure ofgaseous water is lowered proportionally; and (iii) the local temperatureincreases such that the equilibrium point is shifted whereby moreevaporation may occur until the air becomes fully saturated.

Therefore, for a fuel cell 10 operating with excess water at the cathode13 at each location in the fuel cell stack 30, at constant pressure andwith a constant cathode air flow rate, the local equilibrium conditionsare such that the air is fully saturated and any further heat removalthrough evaporation can only be effected by an increase in localtemperature.

The actual increase in temperature to effect evaporation, and thereforecooling, depends on the sensitivity of the equilibrium point forevaporation at the prevailing conditions and the degree of coolingrequired. FIG. 11 shows, schematically, the variation in mass of gaseouswater per unit mass of air with temperature for fully saturatedconditions, i.e. at conditions of 100% relative humidity and at constanttotal pressure. In this way, the operating temperature of the stack islargely set by the amount of evaporation required to effect cooling andthe total pressure and mass flow rate of the cathode air stream.

As shown in FIG. 11, at higher temperatures, a small increase intemperature ΔT leads to a significant increase Δm in the amount ofgaseous water that can be held in the air stream and therefore in theamount of evaporation that can occur as a result of heat generatedwithin the stack. Therefore, the temperature of the stack will remainapproximately constant for a wide range of heat loads, both globally(i.e. as the total stack power is varied) and locally (i.e. due tovariations in local heat generation rate as a consequence ofnon-uniformities within the stack for a given total stack power). Thisgives a high degree of implicit control over the stack operatingtemperature and leads to a good thermal balance being maintained acrossthe stack.

Additionally, the presence of excess water within individual channels 16or passages in the fuel cell stack assembly 30 gives rise to implicitcontrol of the air flow rate within each channel as follows. If a givenpassage has a higher than average air flow rate, then additional watercan be evaporated into the air flow to provide additional cooling ifrequired. This leads to a higher than average volume flow rate at exitfrom the flow passage which, in the presence of a uniform pressure dropacross all flow passages, limits the air flow rate into the cell passageproviding implicit regulation of the air flow leading to improved stackthermal balance and hence improved stack cell voltage balance. A uniformpressure drop across all channels 16 is generally provided by therelative dimensions of the manifolds 21, 22 and the channels 16.

Explicit control of the stack temperature can be achieved if requiredthrough moderation of the cathode air flow rate and/or throughmodification of the total pressure of the cathode air stream. In otherwords, the air flow rate can be increased, thereby lowering the partialpressure of water vapour, by increasing the volume of air into which thewater can vaporise. Therefore, additional water can be vaporised beforesaturation occurs, creating additional cooling and resulting in a lowerstack operating temperature.

Alternatively, or in addition, the outlet pressure can be reduced. Thiswill again lower the partial pressure of water vapour, by lowering thetotal pressure. This has the effect of shifting the equilibrium pointsuch that additional water can be vaporised before saturation isreached, creating additional cooling and resulting in a lower stackoperating temperature.

Other factors, such as the anode flow rate, fuel and oxidant inputtemperature, surface losses etc., are found to be less significant.

In a preferred embodiment of a predominantly unpressurised system, thetypical operating temperature of the fuel cell stack is in the range 70to 80 degrees C. However, in principle, this value can be varied in therange 65 to 95 degrees C. through adjustment of the air flow rate and/ortotal pressure of the cathode air stream. At low power levels, whereevaporative cooling is not dominant, the operating temperature of thestack can be significantly cooler. Operating the system at higher orlower pressures will enable significant variation in the temperatureranges quoted above.

In practice, the average temperature of the reactants and liquid watersupplied to the fuel cell stack can be lower than the stack operatingtemperature. Therefore, some cooling will be provided by the heating ofthese input flows to the stack operating temperature. Once the inputflows have reached the stack operating temperature, the remainder of thecooling will be provided by evaporation of the liquid water into thecathode air stream. The proportion of evaporative cooling is dependenton a number of factors including the cathode air flow rate, the waterflow rate, the fuel cell stack power and the temperature of the inletflows. In most cases, evaporative cooling is the dominant coolingmechanism leading to a high degree of implicit temperature control aspreviously explained. However, for cases where the average temperatureof the inlet flows is lower than the stack operating temperature, therewill be a temperature gradient in the stack in the region where thereactants and liquid water are input.

For evaporative cooling to be effective, there must be sufficient liquidwater present at each part of the fuel cell stack. If insufficient wateris present then the performance of the stack will be reduced withpotentially serious consequences.

Possible problems include: (i) drying of the membrane, leading to alower voltage across the relevant cell; and (ii) hotspots caused by theabsence of liquid water and hence lack of evaporative cooling, leadingto deterioration of the membrane and a reduction in life.

In order to ensure that sufficient liquid water is present to effectcooling through evaporation, alternative strategies can be adopted: (i)precise metering of liquid water to the cathode such that there isexactly enough water to maintain a relative humidity of 100% across theentire surface of the cathode and in each cell of the fuel cell stack;or (ii) over-supply of liquid water to the entire stack such that thereis always excess water present across the entire surface of the cathodeand in each cell of the fuel cell stack. To achieve adequate delivery ofliquid water to the cathode, water injection points may be provided foreach and every cathode channel 16 as will be illustrated later.

In practical fuel cells, the precise metering of liquid water to eachpart of the fuel cell stack is difficult to achieve. Additionally,manufacturing tolerances and non-uniform operating conditions lead todiffering requirements for cooling at each location in the fuel cellstack thereby exacerbating the difficulties associated with precisemetering.

Therefore, the over-supply of liquid water to the cathode such thatthere is always excess liquid water present at the cathode at eachlocation within the stack is the preferred method since this ensuresthat drying of the membrane and hotspots are avoided, leading toimproved stack performance and life.

Thus, in one general aspect, the supply of excess liquid water to thecathode ensures that a relative humidity of 100% is maintainedsubstantially throughout the fluid flow channels in the cathode.

In another aspect, the fuel cell is operated such that, for any measuredcell power delivery, liquid water injection rate into the cathode and/orgas flow through the cathode are controlled to ensure that there is moreliquid water at all regions of the cathode surface than can beevaporated in the prevailing temperature and pressure conditions.

In another aspect, the above conditions are applied to a plurality ofsuch cells in a fuel cell stack having a common oxidant supply manifoldand a common water injection manifold such that, for any measured stackpower delivery, liquid water injection rate into the water injectionmanifold and/or gas flow rate in the oxidant supply manifold arecontrolled to ensure that there is more liquid water at all regions ofthe cathode surfaces of all cells than can be evaporated in theprevailing temperature and pressure conditions.

For a practical stack subject to real non-uniformities and under normaloperating conditions with a water factor of less than unity, it will beappreciated that some parts of the stack could be receiving less liquidwater than is required to maintain a relative humidity of 100%substantially throughout the fluid flow channels in the cathode.Correspondingly, there may be some parts of the stack which arereceiving more liquid water than is required to maintain a relativehumidity of 100% substantially throughout the fluid flow channels in thecathode. Therefore, in another aspect of the invention, the supply ofexcess liquid water to the stack is selected such that all parts of thestack will receive at least the minimum amount of liquid water requiredto maintain a relative humidity of 100% substantially throughout thefluid flow channels in the cathode, corresponding to a water factor ofgreater than unity for the stack.

FIG. 12 shows, schematically, the variation in stack voltage as afunction of the flow rate of liquid water supplied to the cathode for atypical evaporatively cooled fuel cell stack, operating at constantcurrent and constant cathode air flow rate. At low water flow rates, theoverall stack voltage is reduced, indicating that some parts of thestack may not be receiving sufficient liquid water to ensure adequatecooling and/or adequate membrane hydration. As the water flow rate isincreased, a maximum in stack voltage (indicated at label 120) isachieved whereby water is being delivered in excess quantities to allparts of the fuel cell stack. At higher water flow rates, the stackvoltage is gradually reduced, possibly as a consequence of the resultinglower partial pressure of the oxygen in the cathode air stream (beingdisplaced by water) and/or possibly as a consequence of the blockage ofgas transport to or from the membrane by the presence of liquid water.

Also at higher flow rates, the cell balance (as indicated by cellvoltage monitoring) can deteriorate indicating an upper limit to themaximum water factor that can be used for the stack. It is also possiblethat the maximum water flow rate may be set by the maximum achievableusing a suitable water pump.

Despite these limiting effects, it has been determined that a largeoperating window can be defined where significant amounts of excesswater can be supplied to the cathode ensuring adequate hydration andcooling of each part of the fuel cell stack.

It is helpful to describe the quantity of water delivered to the cathodeas a multiple of the theoretical minimum amount required for evaporativecooling, i.e. a ‘water factor’ WF is hereby defined in which:WF=m _(w) /m _(w)(minimum)where m_(w) is the mass flow rate of liquid water delivered, andm_(w)(minimum) is the theoretical minimum mass flow rate of liquid wateras calculated below.

The theoretical minimum amount of water required for evaporative coolingcan be calculated by performing a heat balance on the fuel cell andassuming that:

(i) the enthalpy of reaction is equal to the lower heating value of thefuel, since gaseous water is produced as a product (in the absence ofexcess water);

(ii) the heat load on the fuel cell is derived from an experimentalvalue of fuel cell stack efficiency as a function of stack current;

(iii) the heat load is equal to the thermal enthalpy rise of theproducts over the reactants including complete evaporation of liquidwater supplied to the cathode.

The actual water factor for a given operating point can therefore bedefined as a multiple of this value.

It will be appreciated that the water factor could be defined in otherways than the definition given above, which could result in slightlydifferent preferred ranges of values of water factor, according to thedefinition used.

FIG. 13 shows, schematically, the theoretical minimum liquid water flowrate required as a function of stack current, i.e. a locus of points ofunity water factor, labelled WF=1. As the stack current is increased,the amount of water required increases non-linearly, as the efficiencyof the stack is reduced at higher stack currents leading to a non-linearincrease in the amount of heat generated.

As discussed, stack non-uniformities and the effect of these on waterflow rate for optimum performance (as shown between lines 121 and 122 ofFIG. 12), dictate that a practical fuel cell stack in which separatemetering of water delivery to each location within the stack is notpossible must therefore be operated with a minimum water factor thatallows a margin for the non-uniformities. In other words, the waterfactor used must be sufficiently greater than unity to ensure that allcells in the stack, and all parts of each cell in the stack, achieve100% relative humidity. The maximum water factor used is dictated by amaximum acceptable drop off in performance. Preferred lower and upperlimits of the water factor WF as a function of stack current are shownschematically as dashed lines 130 and 131 in FIG. 13.

The upper and lower limits 130, 131 of water factor may be determinedvia testing or calibration of the relevant fuel cell stack 30.Calibration of a stack can be achieved through variation of the waterflow rate to the cathode operating at constant current and constant airstoichiometry to determine the minimum water flow rates indicated byline 121 and the maximum water flow rates indicated by line 122. Thiscalibration is repeated for a range of possible stack currents (andpossibly also for a range of allowable air stoichiometries) that willcorrespond to a normal range of operating conditions of the stack. Thus,the calibration defines upper and lower limits of water factor as afunction of stack current.

The expression ‘air stoichiometry’ as used herein refers to the amountof oxygen supplied at the input 103 normalised by the amount of oxygenconsumed in the electrochemical reaction. Thus, for an air stoichiometryof 1, all of the oxygen in the air is combined with hydrogen to formwater. For a stoichiometry of 2, 50% of the oxygen is consumed in thecell 10 and 50% is present in the cathode exhaust 101. The amount ofoxygen required for the reaction is a direct function of stack power,stack efficiency and the energy change associated with the reaction.

In a mass manufacturing environment, it is also possible to test anumber of representative stacks in order that a single set of limitswith suitable error margins may be determined that will be acceptablefor all stacks of a given configuration.

In a preferred embodiment, the cathode air flow rate 103 is adjusted inproportion to the stack current such that the stack operates with an airstoichiometry of approximately 2, set by electrochemical requirements.In practice, however, the cathode air flow rate can be varied such thatthe air stoichiometry is within the range 1.1 to 10, and more preferablywithin the range 1.4 to 4, depending on the precise requirements of thefuel cell stack. At low currents, and hence low consumption of reactantsin the cell, the air stoichiometry can be significantly higher thanthese values, since the minimum air flow rate is limited by the minimumflow rate delivery of the air compressor.

In a preferred embodiment, the water flow rate is set to be a linearfunction of stack current as shown schematically in FIG. 13. The waterfactor for this control strategy varies generally in the range 1.5 to40, and more preferably in the range 3 to 6.

In practice, the water factor can be set anywhere in the range 0 to 40depending on the operating conditions of the stack and the acceptablemaximum drop-off in stack performance as a consequence of excess water(refer to FIG. 12). For example, if the stack is operating at low poweroutput or is being started from cold conditions and has therefore notreached its maximum operating temperature, the water flow rate may beset to zero or a low water factor to temporarily increase the rate ofheating of the stack.

Monitoring of the cathode exhaust temperature can be used to indicatethe stack operating temperature and provide feedback control for thewater feed pump. Thus, in one aspect, the system may temporarily permitdelivery of a quantity of liquid water to the fluid flow channels withinthe cathode such that a relative humidity of less than 100% (waterfactor<1) is maintained when the cathode exhaust temperature is below apredetermined threshold corresponding to a sub-optimal operatingtemperature, or for a predetermined period following cold start of thefuel cell.

A metering pump, flow controller or a pressure control method can beused to regulate the water feed rate. At low power levels, the amount ofwater required might be lower than the minimum flow rate obtainable withthe water pump. Therefore, at low power levels a minimum water flow ratecould be set corresponding to the minimum voltage set point for thewater pump to prevent the pump from stalling. This is shownschematically in FIG. 13 for values of current less than I_(crit).

It will be appreciated that the amount of water delivered to the fuelcell stack can, in principle, follow any function of current providedthat the flow rate lies within the minimum and maximum levels of waterfactor determined by calibration of the relevant fuel cell stack or of anumber of representative fuel cell stacks.

Once a water factor control strategy has been defined, additionalflexibility in cooling can be achieved through adjustment of the cathodeair flow rate and/or cathode air total pressure.

Additionally, the stack can be equipped with cell voltage monitoringcapability such that the operational voltage is used as an indicator ofinsufficient or excess water, the necessary adjustments being made inreal time.

An exemplary arrangement for implementing water management in a fuelcell stack is now described in connection with FIG. 14.

A fuel cell system 140 includes a fuel cell stack 30 having a fuel inputline 102, an anode exhaust line 100, an air feed line 103, a waterinjection line 104 and a cathode exhaust line 101. The fuel input lineis fed from a fuel source 141, possibly via a humidifier 142 accordingto well known principles. The anode exhaust line 100 may be fed directlyto ambient 143, or may be at least partially recycled according to knownprinciples using a recycle control loop 144. The air feed line 103 isfed by an air compressor 145. The water injection line 104 is suppliedby a water pump 146. Water can be supplied from an appropriate purifiedwater supply or recycled from the cathode exhaust by way of a suitablecondenser (not shown). The cathode exhaust line 101 may be directed toambient, and preferably includes an exhaust sensor 147 which senses atleast exhaust temperature.

The cathode exhaust may include a pump 148 for reducing and/orcontrolling the cathode exhaust pressure. The pump 148 may be inaddition to, or instead of, a pumped air supply from compressor 145,i.e. the air supply could otherwise be at ambient pressure.

Also included within the fuel cell system 140 is a controller 150 whichpreferably receives sensor inputs corresponding to stack voltage 151,stack current 152 and exhaust temperature 153. The controller 150 isalso coupled to the air compressor 145 and water injection pump 146 byway of suitable control lines.

The controller 150 may be configured to operate in two possible modes.

In a first mode, the controller 150 may be adapted to obtain calibrationdata for subsequent operation of the fuel cell stack 30. In acalibration mode, the controller 150 varies water flow supplied by thewater injection pump 146 under conditions of constant input air pressureand constant current drawn from the fuel cell stack, and receives sensedvoltage levels of the stack to thereby determine appropriate maximum andminimum water factors 121, 122 (FIG. 12). These values are stored incalibration table 154. The calibration may be repeated for one or moreof different current loads, different input air pressures, different airstoichiometries to compile a comprehensive set of control data suitablefor controlling the water injection rate for a range of fuel celloperating conditions.

In a second mode, the controller 150 is adapted to use the calibrationdata stored in calibration table 154 in order to maintain optimumrunning conditions of the fuel cell stack. For example, the controller150 is adapted to monitor stack voltage and current, and to control thewater injection pump 146 (and possibly also the inlet air compressor) tomaintain an appropriate water factor for optimum performance of the fuelcell. In preferred arrangements, this water factor lies in the range 1.5to 40, and more preferably in the range 3 to 6.

As previously described, the controller may also monitor cathode exhausttemperature by way of sensor 147, and deliver a smaller quantity ofwater when the cathode exhaust temperature is below a predeterminedthreshold corresponding to a sub-optimal operating temperature, e.g.during start up of the fuel cell. In another example, this ‘warm-up’phase could be controlled by a timer rather than by exhaust temperature.

In the exemplary embodiment of FIG. 14, the controller is adapted toperform both initial calibration of the fuel cell stack, and maintenanceof optimal running conditions. It will be recognised, however, that forknown fuel cell types, or pre-calibrated systems, the calibration table154 could be preloaded with operating data for use by the watermanagement controller 150.

Although the exemplary embodiment of FIG. 14 shows ‘global’ control ofthe fuel cell stack 30 by the controller 150, it will be understood thata finer granularity of control could be achieved where water delivery todifferent cells, or to different cell groups is possible. For example,where there are multiple, independently controlled water delivery pointsto the fuel cell stack, separate voltage and current sensing may beeffected to locally vary the water delivery to each part of the fuelcell stack.

A number of mechanisms are possible for delivering liquid water inprecisely controlled quantities to fluid flow channels in the cathodefluid flow field plates. Exemplary mechanisms are described inPCT/GB03/02973 (unpublished at the time of filing this application),details of which are described hereinafter with reference to FIGS. 4 to9.

With reference to FIGS. 4( a) and 4(b), the present invention provides aseries of water injection conduits extending between a water inletmanifold 25 and the individual channels 16 of a fluid flow field plate40 a or 40 b. Generally speaking, the water injection conduits areprovided by way of a membrane or laminated structure which lies on thesurface of the fluid flow field plate 40. The water injection conduitsare provided with inlets communicating with the water inlet manifold 25and outlets which define predetermined water injection points over thechannels 16 in the fluid flow field plate.

In a preferred arrangement, the laminated structure is provided in theform of two foil layers 41, 42 overlying the plate 40, the position ofwhich foils are shown in dashed outline in FIGS. 4( a) and 4(b).

FIG. 4( a) illustrates a plan view of a fluid flow field plate 40 a withserpentine channel 16, with foils 41 a, 42 a having first edges 43 a, 44a coincident with the water inlet manifold 25, and second edges 45 a, 46a located at or adjacent to predetermined water injection points 49 ofthe channels 16.

FIG. 4( b) illustrates a plan view of a fluid flow field plate 40 b withtwo interdigitated comb channels 47, 48 each communicating with arespective manifold 21, 22, and foils 41 b, 42 b having first edges 43b, 44 b coincident with the water inlet manifold 25, and second edges 45b, 46 b located at or adjacent to predetermined water injection pointsof the channel 47. It will be noted that the foils may be repeated onthe opposite edge of the plate 40 b between a second water inletmanifold 25 and predetermined water injection points on the channel 48.

FIG. 5 shows a detailed plan view of the water distribution foil 41layout, illustrating the preferred paths of the water injection conduits50. The conduits 50 are formed by a first series of channels 51 whichextend from the first edge 43 of the foil 41 located at the water inletmanifold 25, to a pressure distribution gallery or plenum 52 thatextends along the length of the water injection foil 41. The pressuredistribution gallery 52 communicates with a second series of channels 53which extend to the second edge 45 of the foil for communication withthe channels 16 in the fluid flow field plate. For this purpose, thesecond series of channels 53 are grouped to terminate at respectiveconvergence structures 54 at the second edge 45 of the water injectionfoil 41.

In the preferred embodiment as illustrated, the convergence structures54 comprise arcuate recesses 55 cut into the second edge 45 of the foil41 at water injection points 49 adapted to be coincident withpredetermined positions over channels 16, shown in outline on thefigure.

The pressure distribution gallery 52 preferably comprises an array ofintercommunicating channels 56 which baffle the incoming water from thefirst series of channels 51 and effectively distribute it along theentire length of the foil 41 so that each group of the second series ofchannels 53 receives water at a substantially similar pressure.

Referring back to FIGS. 4( a) and 4(b), the cover foil 42 comprises anunpatterned foil (ie. without channels) of substantially similarperipheral shape to the lower foil. The cover foil 42 extends beyond theedge of the distribution foil 41 at least at the ends of the secondseries of channels to ensure that water is directed downwards into thedesired flow field plate channel 16. Most conveniently, this overlap isachieved by the recesses 55 being formed in the distribution foil 41,but not in the cover foil 42. Thus, as best seen in the cross-sectionaldiagram of FIG. 6, in exaggerated form, the cover foil 42 forms a topclosure to the channels 51, 52 and 53 to form the water injectionconduits 50, leaving open the ends of the channels 51 and 53. In theembodiment shown, the cover foil 42 may be formed slightly larger thanthe distribution foil 41 such that it overlaps the second edge 45 (andpossibly the first edge 43) to achieve a similar effect.

It is noted that the foil layers are very thin compared with the plate40 thickness, the thickness of the foil layers being easily absorbed bythe MEA 34 and any gaskets interposed between the plates. The componentsin FIG. 6 are shown slightly separated for clarity, although they will,of course, be compressed together.

FIG. 7 shows a perspective diagram of the water distribution foil 41 inposition over the flow field plate 40 showing alignment of the variouschannels and manifolds.

It will be recognised that the water distribution channels 51, 52, 53need not be formed in the lower foil 41. In another embodiment, shown inFIG. 8, the water distribution channels 80 are formed in the lowersurface of upper foil 82, while the lower foil 81 serves to form theclosure of the channels 80 to form the water injection conduits. Inother words, the distribution foil 82 and cover foil 81 are invertedcompared with the arrangement of FIG. 6.

In the FIG. 8 arrangement, at least the second series of channels(compare channels 53 in FIG. 5) will not extend right to the second edge83 of the upper foil, but will terminate at positions proximal to thesecond edge. The lower (cover) foil 81 will extend almost to the end ofthe channels 80, but will preferably stop slightly short thereof inorder that there is fluid communication from the end of the channel 80into the plate channel 16 at the water injection points 49.

As indicated above, the lower (cover) foil 81 provides a closure to thechannels 80 forming a barrier preventing water from escaping intounderlying channels 16 in the fluid flow plate 40 in the wrong places,eg. where the water injection conduits traverse the fuel and/or oxidantchannels 16 (eg. at location 85).

Preferably, the foils as described above are formed from a metal, suchas stainless steel. However, any suitable material having appropriatepressurised water containment properties could be used, and theexpression “foil” used throughout the present specification is to beconstrued accordingly. Preferably, the foils are electrically conductivebut they need not be so, since they do not impinge on the active area ofthe MEA.

In a preferred embodiment, the fluid flow channels 16 in the anode orcathode plates 40 are typically between 0.4 mm and 1.2 mm in width anddepth. It is found that a channel width and depth of 10 microns,chemically etched into the water distribution foil, serves to providethe necessary degree of water injection.

In use, the pressure of water being delivered via manifold 25 iscontrolled to ensure a significant pressure difference between the watersupply and the gas pressure in the fluid flow channels 16, achieving anequal distribution of water between the thousands of flow paths. In thepreferred embodiment, water is delivered to the manifold at a gaugepressure in the range 0.5-3 bar H₂O.

An advantage of this approach is that the water distribution membrane isextremely thin and can easily be located within the available spacewithin bipolar plates or in the gasket area.

The volumetric water dispensing accuracy can also be very preciselycontrolled by suitable design of the water injection conduit pattern andchannel dimensions.

As illustrated in FIG. 9, water that is dispensed into interdigitatedchannels 90 in the flow field plate 40 can be introduced at either theentry point 91 to the channel, after the feeder channel 92, oralternatively into the exit track 93 at an injection point 94 at thesame end of the bipolar plate as the feed manifold.

An advantage of water injection into the exit tracks is a reduction ofpressure drop in reactant gas flows. This is because the water does notpass through the diffusion medium causing masking of void space for thegas passage. Similarly the elimination of water flow through thediffusion medium will also reduce the attrition of the medium and itsgradual fragmentation and structural deterioration.

The evaporative cooling process is effective in the exit tracks andwater content of the membrane is maintained due to saturation of the airwith water vapour.

Other embodiments are intentionally within the scope of the accompanyingclaims.

1. A method of operating an electrochemical fuel cell stack comprising aplurality of fuel cells, each of the fuel cells comprising an anode, anion transfer membrane, and a cathode, the method comprising: deliveringfluid fuel to one or more fluid flow channels in each anode of one ormore fuel cells in the electrochemical fuel cell stack; delivering fluidoxidant to one or more fluid flow channels in each cathode of the one ormore fuel cells; exhausting reaction by-products and unused oxidant fromthe one or more fluid flow channels in each cathode of the one or morefuel cells; and delivering a sufficient quantity of liquid water to theone or more fluid flow channels in each cathode of the one or more fuelcells such that a relative humidity of 100% is maintained throughout theone or more fluid flow channels in each cathode of the one or more fuelcells; wherein delivering the sufficient quantity of liquid watercomprises: determining, for each of a plurality of currents, a maximumvoltage for the one or more fuel cells as a function of liquid waterflow rate, the each of a plurality of currents being within a range ofoperating conditions of the one or more fuel cells; determining acalibration function expressing a minimum liquid water flow rate as afunction of current and/or air stoichiometry, the calibration functionbeing determined by variation of water flow rate to the cathodeoperating at constant current and constant air stoichiometry todetermine minimum and maximum water flow rates repeated for a pluralityof stack currents or air stoichiometries; and delivering at least theminimum liquid water flow rate for a corresponding current drawn fromthe one or more fuel cells and/or for the air stoichiometry, thedelivered minimum liquid water flow rate being determined by thecalibration function.
 2. The method of claim 1, wherein the one or morefuel cells comprises less than all fuel cells in the electrochemicalfuel cell stack.
 3. The method of claim 1, wherein the one or more fuelcells comprises all fuel cells in the electrochemical fuel cell stack.4. The method of claim 1, further comprising: increasing a quantity ofliquid water delivered to one or more fluid flow channels of eachcathode of the one or more fuel cells as a function of fuel cell currentin order to maintain a water factor greater than 1.0 for all currentswithin an operating range of the one or more fuel cells.
 5. The methodof claim 1, wherein the calibration function is determined for airstoichiometry in a range 1.1 to
 10. 6. The method of claim 1, whereinthe calibration function is determined for air stoichiometry in a range1.4 to 4.0.
 7. The method of claim 1, wherein delivering the sufficientquantity of liquid water comprises delivery of a water factor of atleast 1.5.
 8. The method of claim 1, wherein delivering the sufficientquantity of liquid water comprises delivery of a water factor of atleast
 3. 9. The method of claim 1, wherein delivering the sufficientquantity of liquid water comprises delivery of a water factor of lessthan
 40. 10. The method of claim 1, wherein delivering the sufficientquantity of liquid water comprises delivery of a water factor in therange from 3 to
 6. 11. The method of claim 1 further comprising:temporarily permitting delivery of a quantity of liquid water to one ormore fluid flow channels of a cathode of the one or more fuel cells suchthat a relative humidity of less than 100% is maintained when an exhausttemperature of the cathode is below a predetermined thresholdcorresponding to a sub-optimal operating temperature.
 12. The method ofclaim 11, which is applied upon start-up of the fuel cell.
 13. Themethod of claim 1, wherein a fuel cell among the one or more fuel cellsis operated such that, for any measured fuel cell power delivery, aliquid water injection rate into a cathode of the fuel cell and/or gasflow through the cathode are controlled to ensure that there is moreliquid water in all regions of a surface of the cathode than can beevaporated in prevailing temperature and pressure conditions.
 14. Themethod of claim 13, which is performed on a plurality of fuel cells inthe electrochemical fuel cell stack having a common oxidant supplymanifold and a common water injection manifold such that, for anymeasured stack power delivery, liquid water injection rate into thecommon water injection manifold and/or gas flow rate in the commonoxidant supply manifold are controlled to ensure that there is moreliquid water in all regions of cathode surfaces of all of the pluralityof fuel cells than can be evaporated in prevailing temperature andpressure conditions.